Gas-sensor diagnosing method

ABSTRACT

Provided is a method of suitably judging necessity of a recovering process carried out on a mixed-potential gas sensor based on an extent of reversible deterioration occurring in a sensing electrode. The method includes the steps of: (a) performing impedance measurement between a sensing electrode exposed to a measurement gas and a reference electrode exposed to a reference atmosphere, which are provided in the gas sensor; and (b) judging necessity of a recovering process based on electrode reaction resistance or a diagnosis parameter correlating with the electrode reaction resistance wherein the electrode reaction resistance and the diagnosis parameter are obtained based on a result of the impedance measurement. The two steps are intermittently or periodically repeated during use of the gas sensor, and it is judged that a recovering process is necessary when the judge parameter satisfies a predetermined threshold condition in the step (b).

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.15/367,334 filed on Dec. 2, 2016, which claims priority under 35 U.S.C.§ 119(a) to Japanese Patent Application No. 2015-244115, filed in Japanon Dec. 15, 2015. The entire contents of U.S. patent application Ser.No. 15/367,334 and the Japanese Patent Application No. 2015-244115 arehereby incorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates to a process of diagnosing a state of amixed-potential gas sensor, and more particularly to a process ofdiagnosing a deterioration state of a sensing electrode of a gas sensor.

Description of the Background Art

Gas sensors that sense a predetermined gas component in a measurementgas such as an exhaust gas, for example, to determine its concentrationcome in various types such as a semiconductor gas sensor, a catalyticcombustion gas sensor, an oxygen-concentration difference sensing gassensor, a limiting current gas sensor, and a mixed-potential gas sensor.Some of these gas sensors are obtained by providing electrodescontaining a noble metal or a metal oxide as its main constituent to asensor element mainly made of ceramic is a solid electrolyte such aszirconia.

As is also well-known, in a gas sensor which includes a sensor elementmainly made of ceramic such as zirconia, a gas component in ameasurement gas or a poisoning substance is adhered to the surface ofthe electrode due to long-term use, or a composing substance of theelectrode is sintered due to exposure of the electrode to ahigh-temperature atmosphere, so that an output value may vary although aconcentration of a gas component to be measured in a measurement gas isconstant.

Among the above-stated causes for output change of a gas sensor,adhesion of a poisoning substance and sintering of a constitutingmaterial of the electrode are irreversible phenomena, and it isconsidered to be difficult to cope directly with change in output valuedue to electrode deterioration (irreversible deterioration) caused bythose phenomena.

On the other hand, it is possible to cope with output change due toadhesion (adsorption) of a gas component in a measurement gas to thesurface of the electrode, by carrying out a predetermined recoveringprocess and removing the adsorbed gas component. That is, such outputchange is caused due to electrode deterioration (reversibledeterioration) caused by a reversible factor. With regard to a gassensor subjected to such reversible deterioration, execution of arecovering process would allow an original (initial) output value to bere-attained, or would allow an output value as close to the originaloutput value as possible to be obtained.

Examples of the foregoing recovering process include an electricalprocess (for example, refer to Japanese Patent Application Laid-Open No.6-265522 (1994) and Japanese Patent No. 3855979), and a heating process(for example, refer to Japanese Patent Application Laid-Open No.11-326266 (1999)).

The electrical process is a method for recovering output by alternatelyapplying positive and negative potentials between electrodes that arepaired through a solid electrolyte, so as to refine the electrode or todesorb an absorbed substance.

On the other hand, the heating process is a method for recovering outputwith exposure of an adsorbed substance or a poisoning substance to ahigh temperature to desorb or burn (oxidize) the substance.

Also, a method of diagnosing presence or absence of an anomalydeterioration for various gas sensors is already known (for example,refer to Japanese Patent No. 4580115, Japanese Patent No. 3855877,Japanese Patent No. 4669369, and Japanese Patent Application Laid-OpenNo. 2014-48279).

Japanese Patent No. 4580115 discloses a technique for judging presenceor absence of an anomaly from a detected value of internal resistance ofa solid electrolytic material forming a gas sensor based on an impedancemodel, in order to prevent a heater from being excessively heated due toan anomaly (increase in resistance) of the solid electrolytic material.

Japanese Patent No. 3855877 discloses a deterioration detectionapparatus which includes air-fuel ratio detection means including asolid electrolytic element, means for detecting deterioration of theair-fuel ratio detection means by comparing output values which arerespectively provided in cases where different temperatures are set forthe solid electrolytic element, temperature control means forcontrolling a temperature of the solid electrolytic element, and meansfor detecting a failure in the temperature control means.

Japanese Patent No. 4669369 discloses an apparatus which periodicallymeasures internal impedance of a sensor element, to judge that a failuresuch as a short circuit or disconnection occurs in the sensor elementwhen a difference value resulted from the periodical measurement exceedsa threshold.

Japanese Patent Application Laid-Open No. 2014-48279 discloses agas-sensor control apparatus which makes an atmosphere within ameasurement chamber included in a detection element of a gas sensor intotwo different states, measures element resistance in the respectivestates, and detects presence or absence of deterioration or an extent ofdeterioration in the detection element based on a magnitude of adifference value between the measured values.

What is a difficult thing in continually using a gas sensor in which theabove-stated output change occurs is to judge what timing is proper forcarrying out a recovering process. This is because, if a recoveringprocess is carried out more frequently than necessary, sintering of amaterial forming an electrode becomes conspicuous unfavorably, whilereduction in a sensor output which is caused due to adsorption of a gascomponent in a measurement gas into a surface of the electrode isprevented. This results in an issue of how reversible deterioration ofan electrode can be appropriately grasped. The reason is that, as longas a recovering process is carried out promptly in a case where it isjudged that an electrode is reversibly deteriorated to an extent thatthe recovering process is necessary, a deteriorated state is canceled,so that a sensor output which is reduced is recovered.

While each of Japanese Patent Application Laid-Open No. 6-265522 (1994),Japanese Patent No. 3855979, and Japanese Patent Application Laid-OpenNo. 11-326266 (1999) discloses a recovering process, none of theabove-cited patent literatures discloses or suggests how a deteriorationstate of an electrode is judged with regard to a gas sensor which iscontinually used.

Also, in a diagnosing method disclosed in each of Japanese Patent No.4580115, Japanese Patent No. 3855877, and Japanese Patent No. 4669369,and Japanese Patent Application Laid-Open No. 2014-48279, there isneither disclosure nor suggestion about determinatioFIGSn of a timing tocarry out a recovering process, based on an extent of reversibledeterioration of an electrode.

SUMMARY OF THE INVENTION

The present invention relates to a method of diagnosing a state of amixed-potential gas sensor, and more particularly to a method ofdiagnosing a deterioration state of a sensing electrode of a gas sensor.

According to the present invention, a gas-sensor diagnosing method ofjudging necessity of a recovering process carried out on amixed-potential gas sensor for recovering an output of the gas sensorincludes the steps of: (a) performing impedance measurement between asensing electrode exposed to an atmosphere of a measurement gas and areference electrode exposed to a reference atmosphere, which areprovided in the gas sensor; and (b) judging necessity of the recoveringprocess based on electrode reaction resistance in the gas sensor or adiagnosis parameter which is a parameter correlated with the electrodereaction resistance, the electrode reaction resistance and the diagnosisparameter being obtained based on a result of the impedance measurement.The step (a) and the step (b) are intermittently or periodicallyrepeated during use of the gas sensor, and it is judged that therecovering process is necessary when the diagnosis parameter satisfies apredetermined threshold condition in the step (b).

Preferably, in the step (a), the impedance measurement is performed byapplication of an alternating voltage between the sensing electrode andthe reference electrode with a frequency being varied within a frequencyrange in which a Nyquist diagram for the electrode reaction resistanceis allowed to be produced, and in the step (b), the electrode reactionresistance is calculated based on the Nyquist diagram produced based ona result of the impedance measurement, and it is judged that therecovering process is necessary when the calculated electrode reactionresistance is equal to, or smaller than, a predetermined threshold.

Alternatively, preferably, the gas-sensor diagnosing method according tothe present invention further includes the step of (c), prior to thestep (a), specifying a diagnosis frequency which is a frequency of analternating voltage used in the impedance measurement in the step (a),by performance of a preliminary impedance measurement in which analternating voltage is applied between the sensing electrode and thereference electrode with a frequency being varied within a predeterminedfrequency range in which a Bode diagram for a phase angle is allowed tobe produced. In the step (a), the impedance measurement is performed byapplication of an alternating voltage at the diagnosis frequency, and inthe step (b), it is judged that the recovering process is necessary whena value of a phase angle which is obtained by the impedance measurementis a value closer to 0° than a predetermined threshold.

Alternatively, preferably, the gas-sensor diagnosing method according tothe present invention further includes the step of (c), prior to thestep (a), specifying a diagnosis frequency which is a frequency of analternating voltage used in the impedance measurement in the step (a),by performance of a preliminary impedance measurement in which analternating voltage is applied between the sensing electrode and thereference electrode with a frequency being varied within a predeterminedfrequency range in which a Bode diagram for an absolute value ofimpedance is allowed to be produced. In the step (a), the impedancemeasurement is performed by application of an alternating voltage at thediagnosis frequency, and in the step (b), it is judged that therecovering process is necessary when a value of a common logarithm of anabsolute value of impedance, which is obtained by the impedancemeasurement, is equal to, or smaller than, a predetermined threshold.

According to the present invention, it is possible to judge necessity ofa recovering process for recovering an output which varies due toreversible deterioration occurring in a sensing electrode of amixed-potential gas sensor based on an extent of the reversibledeterioration occurring in the sensing electrode, so that a recoveringprocess can be carried out at a suitable point in time.

Thus, an object of the present invention is to provide a method ofdiagnosing a mixed-potential gas sensor, which allows necessity of arecovering process for the mixed-potential gas sensor to be suitablyjudged based on an extent of reversible deterioration occurring in asensing electrode.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross-sectional diagrams schematically showing aconfiguration of a gas sensor;

FIGS. 2A and 2B are views schematically showing variation of reactionresistance with time in a mixed-potential gas sensor;

FIG. 3 is a schematic representation of a Nyquist diagram for explainingreaction resistance;

FIG. 4 is a view showing time-series variation of respective Nyquistdiagrams obtained from two-terminal impedance measurement between asensing electrode and a reference electrode which is repeatedlyperformed with the sensing electrode being kept exposed to a gasatmosphere containing an unburned hydrocarbon gas;

FIG. 5 is a view showing time-series variation of Bode diagrams for aphase angle θ of impedance, which are respectively obtained from resultsof impedance measurement performed in order to obtain the Nyquistdiagrams shown in FIG. 4;

FIG. 6 is a view showing time-series variation of Bode diagrams for anabsolute value|Z| of impedance, which are respectively obtained fromresults of impedance measurement performed in order to obtain theNyquist diagrams shown in FIG. 4;

FIG. 7 is a view showing an example of a Bode diagram for an absolutevalue|Z| of impedance; and

FIG. 8 is a view showing variation of Log|Z| with time, for a sensor 1and a sensor 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example of Configuration of aGas Sensor

FIGS. 1A and 1B are cross-sectional diagrams schematically showing aconfiguration of a gas sensor 100A as an example of a diagnosis targetin a diagnosing method according to the present preferred embodiment.The diagnosing method according to the present preferred embodiment is,roughly speaking, a method of judging necessity of a recovering processfor recovering a sensor output which is reduced due to continual use ofthe gas sensor 100A.

FIG. 1A is a vertical sectional view of a sensor element 101A, which isa main component of the gas sensor 100A, taken along the longitudinaldirection of the sensor element 101A. FIG. 1B is a view including across-section of the sensor element 101A perpendicular to thelongitudinal direction of the sensor element 101A at a position A-A′ ofFIG. 1A.

The gas sensor 100A according to the first configuration of the presentinvention is a so-called mixed-potential gas sensor. Generally speaking,the gas sensor 100A determines the concentration of a gas component,which is a measurement target, of a measurement gas using a potentialdifference that occurs between a sensing electrode 10, which is providedon the surface of the sensor element 101A mainly made of ceramic that isan oxygen-ion conductive solid electrolyte such as zirconia (ZrO2), anda reference electrode 20, which is provided inside the sensor element101A, due to a difference in the concentration of the gas componentbetween the portions near the electrodes based on the principle of mixedpotential.

More specifically, the gas sensor 100A preferably determines theconcentration of a predetermined gas component of a measurement gas,where the measurement gas is an exhaust gas present in an exhaust pipeof an internal combustion engine such as a diesel engine or a gasolineengine. In this specification, description will be given of an examplecase where a predetermined gas component being a measurement target isan unburned hydrocarbon gas. In such a case, examples of the unburnedhydrocarbon gas include carbon monoxide (CO) in addition to typicalhydrocarbon gases (gases classified as hydrocarbons in terms of chemicalformula) such as C2H4, C3H6, and n-C8. In the presence of a plurality ofunburned hydrocarbon gases in a measurement gas, a potential differenceoccurring between the sensing electrode 10 and the reference electrode20 is a value reflecting all the plurality of unburned hydrocarbongases, and thus, a concentration value to be determined is also a totalsum of the concentrations of the plurality of unburned hydrocarbongases.

The sensor element 101A mainly includes a reference gas introductionlayer 30, a reference gas introduction space 40, and a surfaceprotective layer 50 in addition to the sensing electrode 10 and thereference electrode 20 described above.

In the present preferred embodiment, the sensor element 101A has thestructure in which six layers, namely, a first solid electrolyte layer1, a second solid electrolyte layer 2, a third solid electrolyte layer3, a fourth solid electrolyte layer 4, a fifth solid electrolyte layer5, and a sixth solid electrolyte layer 6, each formed of an oxygen-ionconductive solid electrolyte, are laminated in the stated order from thebottom side of FIGS. 1A and 1B. The sensor element 101A additionallyincludes other components mainly between these layers or on an outerperipheral surface of the element. The solid electrolytes constitutingthese six layers are fully airtight. Such a sensor element 101A ismanufactured by, for example, laminating ceramic green sheetscorresponding to the individual layers, which have been subjected to apredetermined process and printing of a circuit pattern, and further, byintegrating the laminated layers through firing.

The gas sensor 100A does not necessarily need to include the sensorelement 101A formed of such a laminated body including the six layers.The sensor element 101A may be formed as a laminated body having more orfewer layers or may not have a laminated structure.

In the following description, for convenience' sake, the surface locatedas the upper surface of the sixth solid electrolyte layer 6 in FIGS. 1Aand 1B is referred to as a front surface Sa of the sensor element 101A,and the surface located as the lower surface of the first solidelectrolyte layer 1 in FIGS. 1A and 1B is referred to as a rear surfaceSb of the sensor element 101A. In the determination of the concentrationof the unburned hydrocarbon gas in a measurement gas with the gas sensor100A, a predetermined range starting from a distal end E1 being one endof the sensor element 101A, which includes at least the sensingelectrode 10, is disposed in a measurement gas atmosphere; the otherportion including a base end E2 opposite to the distal end E1 isdisposed so as not to be in contact with the measurement gas atmosphere.

The sensing electrode 10 is an electrode for sensing a measurement gas.The sensing electrode 10 is formed as a porous cermet electrode made ofPt containing a predetermined ratio of Au, namely, Pt—Au alloy andzirconia. The sensing electrode 10 is provided in a substantiallyrectangular shape in plan view at a position closer to the distal end E1that is one end in the longitudinal direction of the sensor element 101Aon the front surface Sa of the sensor element 101A. The gas sensor 100Ais placed such that, in its use, the sensor element 101A correspondingto at least the portion in which the sensing electrode 10 is provided isexposed to a measurement gas.

The catalytic activity of the sensing electrode 10 against combustion ofan unburned hydrocarbon gas is disabled in a predetermined concentrationrange by preferably determining the composition of the Pt—Au alloy beingits constituent material. That is, the combustion reaction of anunburned hydrocarbon gas is prevented or reduced in the sensingelectrode 10. In the gas sensor 100A, accordingly, the potential of thesensing electrode 10 selectively varies with respect to (has correlationwith) the unburned hydrocarbon gas in the concentration range, inaccordance with its concentration. In other words, the sensing electrode10 is provided so as to have high dependence of potential onconcentration for an unburned hydrocarbon gas while having lowdependence of potential on concentration for other components of themeasurement gas.

More specifically, in the sensor element 101A of the gas sensor 100Aaccording to the present preferred embodiment, the sensing electrode 10is provided so as to have a preferably determined Au abundance ratio inthe surface of the Pt—Au alloy particle included in the sensingelectrode 10, thereby exhibiting a strong dependence of potential onconcentration in at least part of a concentration range of 0 ppmC to1000 ppmC, for example. This means that the sensing electrode 10 isprovided to preferably determine the concentration of an unburnedhydrocarbon gas in the foregoing concentration range. For example, whenthe Au abundance ratio is set at 0.7 or more, an unburned hydrocarbongas with a concentration in a range of 4000 ppmC or lower can besuitably sensed. When the Au abundance ratio is set at 0.1 or more andless than 0.7, an unburned hydrocarbon gas with a concentration in arange of 4000 ppmC or higher can be suitably sensed.

In this specification, the Au abundance ratio means an area ratio of aportion covered with Au to a portion at which Pt is exposed in thesurface of noble metal (Pt—Au alloy) particles forming the sensingelectrode 10. In this specification, an Au abundance ratio is calculatedfrom peak intensities of detection peaks for Au and Pt which areobtained by X-ray photoelectron spectroscopy (XPS), with the use of arelative sensitivity coefficient method. When the area of the portionwhere Pt is exposed is equal to the area of the portion covered with Au,the Au abundance ratio is 1.

The reference electrode 20 is an electrode having a substantiallyrectangular shape in plan view, which is provided inside the sensorelement 101A and serves as a reference in the determination of theconcentration of the measurement gas. The reference electrode 20 isprovided as a porous cermet electrode of Pt and zirconia.

It suffices that the reference electrode 20 has a porosity of 10% ormore and 30% or less and a thickness of 5 μm or more and 15 μm or less.The plane size of the reference electrode 20 may be smaller than that ofthe sensing electrode 10 as illustrated in FIGS. 1A and 1B, or may beequal to that of the sensing electrode 10 as in a second configuration,which will be described below (see FIGS. 3A and 3B).

The reference gas introduction layer 30 is a layer made of porousalumina, which is provided inside the sensor element 101A to cover thereference electrode 20. The reference gas introduction space 40 is aninternal space provided near the base end E2 of the sensor element 101A.Air (oxygen), serving as a reference gas in the determination of theconcentration of an unburned hydrocarbon gas, is externally introducedinto the reference gas introduction space 40.

The reference gas introduction space 40 and the reference gasintroduction layer 30 are in communication with each other, andaccordingly, in the use of the gas sensor 100A, the surrounding of thereference electrode 20 is always filled with air (oxygen) through thereference gas introduction space 40 and the reference gas introductionlayer 30. During the use of the gas sensor 100A, thus, the referenceelectrode 20 always has a constant potential.

The reference gas introduction space 40 and the reference gasintroduction layer 30 are provided so as not to come into contact with ameasurement gas owing to their surrounding solid electrolytes. Thisprevents the reference electrode 20 from coming into contact with themeasurement gas even when the sensing electrode 10 is exposed to themeasurement gas.

In the case illustrated in FIGS. 1A and 1B, the reference gasintroduction space 40 is provided in such a manner that part of thefifth solid electrolyte layer 5 is in communication with the exterior onthe base end E2 of the sensor element 101A. The reference gasintroduction layer 30 is provided so as to extend in the longitudinaldirection of the sensor element 101A between the fifth solid electrolytelayer 5 and the sixth solid electrolyte layer 6. The reference electrode20 is provided under the center of gravity of the sensing electrode 10with reference to FIGS. 1A and 1B.

The surface protective layer 50 is a porous layer made of alumina, whichis provided so as to cover at least the sensing electrode 10 on thefront surface Sa of the sensor element 101A. The surface protectivelayer 50 is provided as an electrode protective layer that prevents orreduces the degradation of the sensing electrode 10 due to continuousexposure to a measurement gas during the use of the gas sensor 100A. Inthe case illustrated in FIG. 1A, the surface protective layer 50 isprovided so as to cover not only the sensing electrode 10 but alsosubstantially the entire front surface Sa of the sensor element 101Aexcept for a predetermined range starting from the distal end E1.

As illustrated in FIG. 1B, the gas sensor 100A is equipped with apotentiometer 60 capable of measuring a potential difference between thesensing electrode 10 and the reference electrode 20. Although FIG. 1Bschematically illustrates wiring of the sensing electrode 10, thereference electrode 20, and the potentiometer 60, in an actual sensorelement 101A, connection terminals (not shown) are providedcorrespondingly to the respective electrodes on the front surface Sa orthe rear surface Sb on the base end E2 side, and wiring patterns (notshown), which connect the respective electrodes and their correspondingconnection terminals, are formed on the front surface Sa and inside theelement. The sensing electrode 10 and the reference electrode 20 areelectrically connected with the potentiometer 60 through the wiringpatterns and the connection terminals. Hereinbelow, a potentialdifference between the sensing electrode 10 and the reference electrode20, which is measured by the potentiometer 60, is also referred to as asensor output.

The sensor element 101A further includes a heater part 70, whichperforms temperature control of heating the sensor element 101A andmaintaining the temperature of the sensor element 101A, to enhance theoxygen ion conductivity of the solid electrolyte. The heater part 70includes a heater electrode 71, a heater 72, a through hole 73, a heaterinsulating layer 74, and a pressure diffusion hole 75.

The heater electrode 71 is an electrode formed while being in contactwith the rear surface Sb of the sensor element 101A (in FIGS. 1A and 1B,the lower surface of the first solid electrolyte layer 1). The heaterpart 70 can be powered externally by the heater electrode 71 connectedwith an external power supply (not shown).

The heater 72 is an electric resistor provided inside the sensor element101A. The heater 72 is connected with the heater electrode 71 throughthe through hole 73 and generates heat by being powered externally viathe heater electrode 71 to heat the solid electrolytes forming thesensor element 101A and maintain their temperatures.

In the case illustrated in FIGS. 1A and 1B, the heater 72 is buriedwhile being vertically sandwiched between the second solid electrolytelayer 2 and the third solid electrolyte layer 3 so as to extend from thebase end E2 to the position below the sensing electrode 10 near thedistal end E1. This enables the adjustment of the entire sensor element101A to the temperature at which the solid electrolytes are activated.

The heater insulating layer 74 is an insulating layer formed of aninsulator such as alumina on the upper and lower surfaces of the heater72. The heater insulating layer 74 is formed for electrical insulationbetween the second solid electrolyte layer 2 and the heater 72 and forelectrical insulation between the third solid electrolyte layer 3 andthe heater 72.

The pressure diffusion hole 75 is a part provided to penetrate the thirdsolid electrolyte layer 3 and the fourth solid electrolyte layer 4 andto be in communication with the reference gas introduction space 40, andis formed to mitigate an internal pressure rise associated with atemperature rise in the heater insulating layer 74.

In the determination of the concentration of an unburned hydrocarbon gasin a measurement gas using the gas sensor 100A having such aconfiguration, as described above, air (oxygen) is supplied to thereference gas introduction space 40, with the sensor element 101A inonly a predetermined range, which starts from the distal end E1 andincludes at least the sensing electrode 10, being disposed in a spacecontaining a measurement gas, and with the sensor element 101A on thebase end E2 being apart from the space. The heater 72 heats the sensorelement 101A to an appropriate temperature from 400° C. to 800° C.,preferably from 500° C. to 700° C., more preferably from 500° C. to 600°C.

In such a state, a potential difference occurs between the sensingelectrode 10 exposed to the measurement gas and the reference electrode20 exposed to the air. As described above, however, the potential of thereference electrode 20 disposed in the air (having a constant oxygenconcentration) atmosphere is maintained at a constant potential, whereasthe potential of the sensing electrode 10 selectively has a dependenceon concentration for the unburned hydrocarbon gas of the measurementgas. The potential difference (sensor output) is thus substantially avalue according to the composition of the measurement gas present aroundthe sensing electrode 10. Therefore, a certain functional relationship(referred to as sensitivity characteristics) holds between theconcentration of the unburned hydrocarbon gas and the sensor output. Inthe description below, such sensitivity characteristics may also bereferred to as, for example, sensitivity characteristics for the sensingelectrode 10.

In the actual determination of the concentration of an unburnedhydrocarbon gas, in advance, a plurality of different mixed gases, eachof which has a known concentration of an unburned hydrocarbon gas, areused as the measurement gas, and the sensitivity characteristics areexperimentally identified by performing a measurement on the sensoroutput for each measurement gas. In the actual use of the gas sensor100A, accordingly, an operation processor (not shown) converts thesensor output, which varies from moment to moment in accordance with theconcentration of an unburned hydrocarbon gas in a measurement gas, intothe concentration of the unburned hydrocarbon gas based on thesensitivity characteristics. The concentration of the unburnedhydrocarbon gas in the measurement gas can thus be determined almost inreal time.

Aging Variation of Electrode Reaction Resistance, and Recovering Process

FIGS. 2A and 2B are views schematically showing variation of reactionresistance (electrode reaction resistance) with time in amixed-potential gas sensor like the gas sensor 100A.

Conventionally, during continual use of a mixed-potential gas sensor,reaction resistance has a tendency to be reduced with time as indicatedby a straight line L1 in FIG. 2A. Such reduction in reaction resistancecauses a sensor output to be reduced with time.

Reaction resistance is obtained from a Nyquist diagram on which a resultof two-terminal impedance measurement performed by application of analternating voltage between a sensing electrode and a referenceelectrode in a gas sensor with a frequency being varied is plotted, witha horizontal axis being defined as a real axis (R′ axis, unit: Ω) and avertical axis being defined as an imaginary axis (R″ axis, unit: Ω).FIG. 3 is a schematic representation of a Nyquist diagram for explainingderivation of reaction resistance.

More specifically, as a result of plotting of actual-measurement data, acurve in a shape of an arc having an origin on one side at a point on areal axis (R′, R″)=(R1, 0) is provided as shown in FIG. 3. Then, when R′coordinate value of an end point on the other side of the curve, or R′coordinate value of an extrapolation point extrapolated from the endpoint on the real axis R′, is expressed as R1+R2, an increment R2 withrespect to the R′ coordinate value R1 is reaction resistance.Additionally, the value R1 is IR resistance (insulation resistance),which corresponds to material resistance of a solid electrolyte forminga sensor element, for example, in a case of a mixed-potential gas sensorlike the gas sensor 100A. Accordingly, if an anomaly occurs in a solidelectrolyte, not a value R2 but a value R1 varies.

The inventors of the present invention have found that whereas adhesion(adsorption) of a gas component in a measurement gas to a surface of asensing electrode has a tendency to reduce reaction resistance with timeas indicated by a straight line La in FIG. 2A, an irreversiblephenomenon (irreversible deterioration) such as adhesion of a poisoningsubstance or sintering of a material forming a sensing electrode has atendency to increase reaction resistance with time as indicated by astraight line Lb in FIG. 2A, while diligently investigating and studyingthe above-described reduction in reaction resistance and sensor outputwith time. Further, the inventors have acquired knowledge that thereason why actual reaction resistance is reduced with time as indicatedby the straight line L1 and a sensor output is accordingly reduced withtime is that reduction in reaction resistance due to adsorption of a gascomponent indicated by the straight line La is steeper than, andtherefore dominant over, increase in reaction resistance due to anirreversible phenomenon indicated by the straight line Lb. Additionally,while variation of reaction resistance with time is indicated bystraight lines in FIGS. 2A and 2B in order to represent tendencies ofincrease and reduction in a simplified manner, actual variation withtime does not necessarily occur linearly.

Conventionally-performed known recovering processes such as anelectrical process disclosed in Japanese Patent Application Laid-OpenNo. 6-265522 (1994) and Japanese Patent No. 3855979 and a heatingprocess disclosed in Japanese Patent Application Laid-Open No. 11-326266(1999), for example, are intended for cancellation of reduction insensor output which is caused due to adsorption of a gas component intoa surface of a sensing electrode, out of the above-described reductions.

In FIG. 2B, a solid line L2 indicates variation of reaction resistancewith time in a case where a recovering process is carried out at somemidpoint during use of a gas sensor. In the case shown in FIG. 2B, likethe case shown in FIG. 2A, reaction resistance having an initial valueR20 is reduced with time along a line segment L21 which is defined by abalance between the straight line La indicating reduction in reactionresistance caused due to adsorption (reversible deterioration) of a gascomponent into a surface of a sensing electrode and the straight line Lbindicating increase in reaction resistance caused due to irreversibledeterioration in a gas sensor. For example, if a recovering process P1is carried out at a point in time when reaction resistance has a certainvalue R21 as shown in FIG. 2B, decrement caused due to adsorption of agas component is canceled, and a sensor output is recovered. However,also irreversible deterioration proceeds with time in a sensingelectrode, so that reaction resistance has a value R22 on the straightline Lb, not the initial value R20, after a recovering process. Thismeans that a recovering process achieves the same state as a state inwhich only irreversible deterioration occurs with time.

After that, as a gas sensor continues to be used, a value of reactionresistance is reduced with time along a line segment L22 defined by abalance between the straight line La and the straight line Lb again.However, if a recovering process P2 is carried out again at a point intime when reaction resistance has a value R23, decrement caused due toadsorption of a gas component is canceled again, and reaction resistanceis recovered to a value R24 on the straight line Lb.

Thereafter, by repetition of a recovering process in a similar manner atappropriate points in time, reaction resistance, together with a sensoroutput, which is once reduced, is recovered again after each recoveringprocess. Moreover, a state of a sensing electrode after each recoveringprocess is equivalent to a state where only irreversible deteriorationproceeds. In other words, reduction in reaction resistance with timeafter a recovering process is once carried out starts from a state whereonly irreversible deterioration proceeds.

However, depending on a way in which a gas sensor is actually used,irreversible deterioration may proceed only at an extremely moderatepace, as compared to reversible deterioration. In those cases, a stateof a sensing electrode after a recovering process can be regarded asbeing almost identical to an initial state.

Method of Judging Necessity of Recovering Process

Next, a method of judging necessity of recovering process will bedescribed. Since a recovering process is intended to recover a sensoroutput which is reduced with time, necessity of a recovering process maybe judged based on a behavior of a sensor output. In this regard, asdescribed above, a process carried out as a recovering process itselfmay cancel reduction in reaction resistance causing reduction in sensoroutput, with elimination of adsorption of a gas component into a sensingelectrode, i.e., elimination of reversible deterioration. However,variation in sensor output is also affected by irreversibledeterioration which is not a target of the recovering process andincreases reaction resistance with time. Accordingly, in order toachieve elimination of reversible deterioration which is an originaleffect provided by a recovering process, at a suitable timing, it ispreferable to carry out the recovering process based on a value ofreaction resistance which is reduced, reflecting a deterioration stateof a sensing electrode which is caused due to reversible deterioration.

Moreover, it is not preferable to carry out a recovering processexcessively, because sintering of a material forming a sensing electrodewould be promoted, so that increase in reaction resistance which iscaused due to irreversible deterioration, which is shown as beingmoderate in FIGS. 2A and 2B, may become much steeper. On the other hand,it is not preferable to carry out a recovering process at excessivelylong intervals, because reaction resistance, as well as a sensor output,would be significantly reduced, resulting in impaired measurementaccuracy.

In view of the foregoing matters, a method of judging necessity of arecovering process according to the present preferred embodimentincludes three manners as follows.

(First Manner: Judgement Based on Nyquist Diagram)

As schematically shown in FIG. 3, reaction resistance is specified froma Nyquist diagram obtained based on a result of two-terminal impedancemeasurement in which an alternating voltage is applied between a sensingelectrode and a reference electrode in a gas sensor with a frequencybeing varied. Accordingly, if a value of reaction resistance obtainedfrom a Nyquist diagram is below a predetermined threshold, it indicatesthat deposition of a gas component on a sensing electrode proceeds to anextent of affecting measurement accuracy, so that it is judged that arecovering process is necessary.

FIG. 4 is a view showing time-series variation of respective Nyquistdiagrams obtained when two-terminal impedance measurement between thesensing electrode 10 and the reference electrode 20 which isperiodically repeated while the sensing electrode 10 is kept exposed toa gas atmosphere containing an unburned hydrocarbon gas and the gassensor 100A is driven with a temperature of a sensor element being setat 600° C. Conditions of measurement are as follows.

Gas atmosphere (produced with the use of a model gas apparatus):

C₂H₄ (corresponding to an unburned hydrocarbon gas)=1000 ppm (=2000ppmC);

O₂O=10%; H₂O=5%;

N₂=residual;Impedance measurement:

Frequency=0.1 Hz to 1 MHz; Amplitude=10 mV;

Bias voltage for open-circuit voltage (OCV)=0;Measurement interval . . . at five-minute intervals in sixty minutesfrom a time of gas introduction (t=5 to 60)

Additionally, a concentration of C₂H₄ in a gas atmosphere is set at 2000ppmC which is higher than a concentration of an unburned hydrocarbon gasin an exhaust gas exhausted from a general internal combustion engine,in order to accelerate adsorption of C₂H₄ into the sensing electrode 10.Also, a measurement interval for impedance measurement is set at everyfive minutes in accordance with such acceleration of adsorption, and alonger measurement interval may be set for actual use of a gas sensor.

From FIG. 4, it can be seen that whereas the respective Nyquist diagramsare similar to one another in the respect that a curve in a shape of anarc starting from a position of approximately 1150 (Ω) on a real axis isformed, a coordinate value on a real axis of the other end point of thecurve has a tendency to decrease from a Nyquist diagram for the firstimpedance measurement at t=5 to a Nyquist diagram for the last impedancemeasurement at t=60.

By using the foregoing tendency, it is possible to judge that therecomes a timing to carry out a recovering process on a gas sensor.Namely, a predetermined threshold for a value of reaction resistancewhich corresponds to R2 in FIG. 3 and is obtained by extrapolation ofthe curve on a real axis is determined in advance, and a value ofreaction resistance is intermittently (or periodically) and repeatedlyobtained at predetermined intervals by performance of impedancemeasurement during use of a gas sensor. When a value of reactionresistance as obtained falls below the threshold, it is judged that thetiming has come. This is the method of judging necessity of a recoveringprocess in this manner. Since only a value corresponding to R2 in FIG. 3is to be used for diagnosis target (diagnosis parameter), it is possibleto achieve diagnosis targeted only to resistance variation which occursby a reversible cause and can be recovered by a recovering process.

Additionally, the threshold may be set at a value which is obtained frommultiplication of a value of reaction resistance in the initial stage ofuse by a predetermined threshold coefficient α(0<α<1) until the firstrecovering process is carried out from a start of use of a gas sensor.After a recovering process is carried out, the threshold may be set at avalue which is obtained from multiplication of the value of reactionresistance obtained by performance of impedance measurement immediatelyafter the recovering process by the threshold coefficient α. As a resultof this, after a recovering process is once carried out, necessity tocarry out a recovering process again can be appropriately judged withreference to a state provided after the recovering process. A specificvalue of the threshold coefficient α can be determined appropriately,considering measurement accuracy which is required of a gas sensor, acomposition of a sensing electrode, and the like.

(Second Manner: Judgment Based on Phase Angle)

In the above-described first manner of judgment based on a Nyquistdiagram, it is required to measure impedance over a wide frequency range(a range of 0.1 Hz to 1 MHz in the case shown in FIG. 4) every time anopportunity to perform impedance measurement comes, because reactionresistance is directly obtained. Accordingly, a considerable time isrequired to perform impedance measurement once. Also, as generallyknown, a concentration of a component in an exhaust gas exhausted froman internal combustion engine is likely to vary. Thus, strictlyspeaking, spending a considerable time on measurement may unfavorablycause a concentration of an unburned hydrocarbon gas in a measurementtarget to vary by a frequency in some cases.

On the other hand, what is essentially required to judge necessity of arecovering process is to grasp a state in which reaction resistance isreduced to fall out of a predetermined tolerable range. Grasp of such astate is not necessarily achieved by directly obtaining a value ofreaction resistance. Necessity of a recovering process may be judged byusing a parameter correlated with reaction resistance as a diagnosisparameter, instead of reaction resistance itself.

From this point of view, in this manner, a phase angle resulted fromimpedance measurement is used for judgment.

FIG. 5 is a view showing time-series variation of Bode diagrams for aphase angle θ of impedance, which are respectively obtained from resultsof impedance measurement performed in order to obtain the Nyquistdiagrams shown in FIG. 4

From FIG. 5, it can be seen that each of the Bode diagrams has a peakvalue (extremum) when a frequency is in a range of approximately 7 Hz to10 Hz, and that a peak value (extremum) has a tendency to get near tozero from a Bode diagram for the first impedance measurement at t=5, toa Bode diagram for the last impedance measurement at t=60. Such atendency is correlated with a tendency of reaction resistance to bereduced, which is represented in the Nyquist diagrams shown in FIG. 4.

Thus, according to the present way, impedance measurement isexperimentally performed in advance or in the initial stage of actualuse of a gas sensor, regarding a predetermined frequency range which iswide enough to produce a Bode diagram for a phase angle θ. Then, basedon a result of the impedance measurement, a Bode diagram for a phaseangle θ is produced, and a frequency contributing to a peak value(extremum) is designated as a frequency used for diagnosis (which willbe referred to as a “diagnosis frequency”). Also, a threshold of a phaseangle θ at the diagnosis frequency is determined. Such impedancemeasurement is regarded as preliminary measurement in the present way.

Then, in actual use of a gas sensor, only impedance measurement in whichan alternating voltage at a diagnosis frequency is applied is performedat predetermined intervals. When a value of a phase angle θ which isobtained from a result of the foregoing impedance measurement becomescloser to zero than a threshold, it can be judged that there comes atiming to carry out a recovering process on a gas sensor.

The reason for the foregoing matter is as follows. Reduction in reactionresistance and variation in peak value (extremum) in a Bode diagram fora phase angle θ are correlated with each other as described above, andso, by determining a threshold of a phase angle θ in this manner so asto be commensurate with a threshold of reaction resistance in the firstmanner, it is possible to judge that reaction resistance is reduced toan extent that a recovering process is necessary, without directlyobtaining reaction resistance. This is a method of judging necessity ofa recovering process in this manner.

Preferably, impedance measurement is performed over a wide frequencyrange (for example, a range of 0.1 Hz to 1 MHz) immediately after arecovering process, and a peak value (extremum) in a Bode diagram for aphase angle θ is obtained. Then, a threshold is set in accordance withthe peak value (extremum). As a result of this, also in this manner, aswell as the first manner, necessity to carry out a recovering processagain can be appropriately judged with reference to a state providedafter the recovering process.

This manner, in which only one diagnosis frequency serves as ameasurement frequency used in impedance measurement performed at anappropriate point in time during actual use of a gas sensor, isadvantageous over the first manner in that a time required to judgenecessity of a recovering process is shortened. Because of a shortertime for measurement, for example, in a case where a gas sensor isattached to an exhaust pipe of an automobile and an exhaust gas is ameasurement gas, it is preferable to carry out a diagnosing processunder a condition that components of a measurement gas are settled,namely, immediately before engine starting, immediately after enginestarting, at an idling time, at a fuel-cut time, or the like. This couldfavorably improve diagnosis accuracy.

Additionally, in FIG. 5, a frequency contributing an extremum has atendency to increase as a value of t increases. In a case where such atendency is empirically identified, a diagnosis frequency may be set tobe slightly higher (by approximately 2 Hz in the case shown in FIG. 5)than a frequency contributing to a peak value (extremum) in a Bodediagram for a firstly-obtained phase angle θ.

(Third Manner: Judgement Based on an Absolute Value of Impedance)

According to the above-described second manner, necessity of arecovering process is judged by utilizing a peak occurring in a Bodediagram for a phase angle θ. Instead of that, necessity of a recoveringprocess can be judged based on a Bode diagram for an absolute value|Z|of impedance.

FIG. 6 is a view showing time-series variation of Bode diagrams for anabsolute value|Z| of impedance, which are respectively obtained fromresults of impedance measurement performed in order to obtain theNyquist diagrams shown in FIG. 4. It is noted that a vertical axis inFIG. 6 represents a common logarithm Log|Z| of an absolute value|Z|.

From FIG. 6, it can be seen that each of the Bode diagrams has atendency to substantially monotonously decrease as a frequencyincreases. Further, all of the Bode diagrams substantially agree withone another when a frequency is in a range of approximately 10 Hz orhigher.

On the other hand, when a frequency is in a range of 10 Hz or lower, avalue of Log|Z| has a tendency to decrease with elapse of time from aBode diagram for the first impedance measurement at t=5 to a Bodediagram for the last impedance measurement at t=60. Such a tendency iscorrelated with a tendency of reaction resistance to be reduced, whichis represented in the Nyquist diagrams shown in FIG. 4.

From this point of view, in this manner, impedance measurement isexperimentally performed in advance or in the initial stage of actualuse of a gas sensor, regarding a predetermined frequency range which iswide enough to produce a Bode diagram for an absolute value|Z| ofimpedance. Then, based on a result of the impedance measurement, a Bodediagram for an absolute value|Z| is produced. Such impedance measurementis regarded as preliminary measurement in the present way. FIG. 7 showsan example of the foregoing Bode diagram. Further, a value of aparameter such as a frequency used for diagnosis (which will be referredto as a “diagnosis frequency”) is previously specified based on theforegoing Bode diagram.

More specifically, when the Bode diagram as shown in FIG. 7 is obtainedby impedance measurement, a frequency at which an absolute value|Z| isconsidered to be substantially unchanging in the Bode diagram despiteelapse of time (which will be hereinafter referred to as a “referencefrequency”) is specified. Also, one of frequencies in the vicinity of afrequency which contributes to a maximum of Log|Z| is designated as adiagnosis frequency. Further, a value Log|Z1| of Log|Z| at a referencefrequency and a value Log|Z2| of Log|Z| at a diagnosis frequency arespecified.

In the case shown in FIG. 7, a reference frequency is set at 100 Hz, anda diagnosis frequency is set at 1 Hz. As a result of this, Log|Z1| isequal to 4.15, and Log|Z2| is equal to 4.85.

Further, a threshold coefficient k by which a value of differencebetween Log|Z1| and Log|Z2|, i.e., ΔLog|Z|=Log|Z2|−Log|Z1|, ismultiplied, is determined. In this regard, a threshold coefficient k isa value satisfying 0<k<1, and a value which indicates to what extentdecrease of Log|Z| which is caused as a gas sensor continues to be usedas shown in FIG. 6 can be allowed with respect to a value of differenceΔLog|Z|. By determining the threshold coefficient k such that anallowable decrement of Log|Z| is commensurate with a threshold ofreaction resistance in the first manner, it is possible to judge thatreaction resistance is reduced to an extent that a recovering process isnecessary, without directly obtaining reaction resistance.

Actually, during actual use of a gas sensor, only impedance measurementin which an alternating voltage at a diagnosis frequency is applied isperformed. Then, when a value of Log|Z| at the diagnosis frequencybecomes equal to, or smaller than, a threshold determined by anexpression of TH=Log|Z1|+k·ΔLog|Z|, it is judged that a recoveringprocess is necessary. The case shown in FIG. 7 is an example where thethreshold coefficient k is set at 0.8 (80%). Also,ΔLog|Z|=Log|Z2|−Log|Z1| is equal to 0.7. Accordingly, when a value ofLog|Z| at a frequency of 1 Hz which is a diagnosis frequency falls belowTH=4.15+0.8×0.7=4.71, it is judged that a recovering process isnecessary.

For example, when values of Log|Z| at t=t1, t2 (t1<t2) are representedas Log|Z (t=t1)| and Log|Z(t=t2)|, respectively, the followingexpressions are formulated in the case shown in FIG. 7:Log|Z(t=t1)|=4.8>TH; and Log|Z=t2)|=4.7<TH. Thus, it is judged thatwhile a recovering process is unnecessary at t=t1, a recovering processis necessary at t=t2.

Preferably, impedance measurement is performed over a wide frequencyrange (for example, a range of 0.1 Hz to 1 MHz) immediately after arecovering process, and a Bode diagram for an absolute value|Z| ofimpedance is obtained. Then, a value of a threshold TH is determinedbased on the Bode diagram as obtained. As a result of this, also in thismanner, like the first manner, necessity to carry out a recoveringprocess again can be appropriately judged with reference to a stateprovided after the recovering process.

This manner, like the second manner, in which only one diagnosisfrequency serves as a measurement frequency used in impedancemeasurement performed at an appropriate point in time during actual useof a gas sensor, is advantageous over the first manner in that a timerequired to judge necessity of a recovering process is shortened.Because of a shorter time for measurement, in this manner, also like thesecond manner, it is preferable to carry out a diagnosing process undera condition that components of a measurement gas are settled. This couldfavorably improve diagnosis accuracy.

As described above, according to the present preferred embodiment,necessity of a recovering process for recovering an output which variesdue to reversible deterioration occurring in a sensing electrode of amixed-potential gas sensor can be judged based on an extent of thereversible deterioration occurring in the sensing electrode. Therefore,it is possible to carry out a recovering process at a suitable timing.

MODIFICATIONS

While in the above-described preferred embodiment, only the gas sensor100A is described as an example of a gas sensor to be diagnosed, a gassensor having another configuration can be a target to be diagnosed.

Though a diagnosing process is carried out based on a result ofimpedance measurement in which an alternating voltage is applied betweena sensing electrode and a reference electrode in the above-describedpreferred embodiment, a diagnosing process may be carried outalternatively based on resistance variation which is caused when adirect voltage is applied (variation in resistance by direct-currentmeasurement). For example, a current flowing when 0 V is applied betweena sensing electrode and a reference electrode and a current flowing when0.1 V is applied between those electrodes are measured, and resistance(direct-current resistance) is obtained from a slope of avoltage-current line (VI line) which is specified based on results ofmeasurement of the two currents. Then, a diagnosing process may becarried out in such a manner that it is judged that a recovering processis necessary when the resistance as obtained becomes equal to, orsmaller than, a predetermined threshold, or the like manner.

While the above-described preferred embodiment deals with a case wherean exhaust gas present within an exhaust pipe of an internal combustionengine such as a diesel engine or a gasoline engine is a measurement gasand a concentration of an unburned hydrocarbon gas in the measurementgas is determined, a target to be measured by the gas sensor 100A is notlimited to a hydrocarbon gas. The gas sensor 100A can measure also NH₃and NO_(x) based on principles of mixed potential in a similar way tothe above-described ways in the above-described preferred embodiment.

Also, while each of a sensing electrode and a reference electrode isprovided as a cermet electrode formed of a noble metal (morespecifically, Pt—Au alloy for a sensing electrode and Pt for a referenceelectrode) and zirconia in the above-described preferred embodiment, asensing electrode may alternatively be formed of an oxide of at leastone kind of metals including Cu, Zn, Sn, La, Nb, Sr, Ti, Si, Cr, In, Cd,Ni, W, V, Fe, Tb, Bi, Ta, Y, Ga, Mo, and Co, or a complex oxide which isa mixture of some of oxides of those metals. Those oxides and complexoxides do not serve as a combustion reaction catalyst of a measurementgas, so that electrochemical reaction is caused in a three-phaseinterface.

EXAMPLE

With the use of two different gas sensors 100A (which will behereinafter referred to as a “sensor No. 1”, and a “sensor No. 2”),effects of a recovering process based on a diagnosing process wereconfirmed.

More specifically, each of the two gas sensors 100A was continuouslydriven while the sensing electrode 10 was kept exposed to a gasatmosphere containing an unburned hydrocarbon gas and a temperature of asensor element was set at 600° C., and a diagnosing process was carriedout according to the third manner. Then, a procedure of carrying out arecovering process immediately after it was judged that a recoveringprocess was necessary was repeated.

A gas atmosphere is as follows (produced with the use of a model gasapparatus):

C₂H₄ (corresponding to an unburned hydrocarbon gas)=2000 ppm (4000ppmC);

O₂=10%; H₂O=5%; and

N₂=residual.

A diagnosis frequency was set at 1 Hz, and impedance measurement for adiagnosing process was performed with the use of a known impedanceanalyzer at 100-second intervals (at 100-second intervals after arecovering process in a case where a recovering process was carriedout). Also, impedance measurement had previously been performed in arange of 0.1 Hz to 1 MHz, so that the initial threshold TH was set at4.82 for the sensor No. 1, and the initial threshold TH was set at 4.80for the sensor No. 2.

Additionally, a concentration of C₂H₄ in the gas atmosphere was set at4000 ppmC which was higher than a concentration of an unburnedhydrocarbon gas in an exhaust gas exhausted from a general internalcombustion engine, in order to accelerate adsorption of C₂H₄ into thesensing electrode 10. Also, a measurement interval for impedancemeasurement was set at every 100 seconds in accordance with suchacceleration of adsorption, and a longer measurement interval may be setfor actual use of a gas sensor.

A recovering process was carried out with the sensor element 101A beingheated for 30 seconds at a heating temperature of 850° C. by the heaterpart 70.

FIG. 8 is a view showing time-series variation of a logarithm Log|Z| ofan absolute value|Z| of impedance for each of the sensors No. 1 and thesensor No. 2 after a start of driving the sensors. It is noted that thesensor No. 1 and the sensor No. 2 are depicted simply as “SENSOR 1” and“SENSOR 2”, respectively, in FIG. 8. The initial values of Log|Z| of thegas sensors 100A are 4.90 and 4.82, respectively.

In each of the sensor No. 1 and the sensor No. 2, a value of Log|Z|became equal to, or smaller than, a value TH thereof when 200 secondshad passed, and immediately after that, a recovering process was carriedout at a point in time indicated by an arrow in FIG. 8. Immediatelyafter the recovering process, impedance measurement was performed, whichindicated that Log|Z| was identical to the initial value. Accordingly,the respective values of thresholds TH for the sensor No. 1 and thesensor No. 2 were set at values identical to the respective initialvalues.

Thereafter, in the same manner as described above, a diagnosing processat 100-second intervals and a procedure of carrying out a recoveringprocess at a point in time indicated by an arrow in FIG. 8 when thevalue of Log|Z| became equal to, or smaller than, a value of TH, wererepeated. In a range of up to 900 seconds shown in FIG. 8, a value ofLog|Z| provided after a recovering process agreed with an initial value.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A gas-sensor diagnosing method of judgingnecessity of a recovering process carried out on a mixed-potential gassensor for recovering an output of said gas sensor, said methodcomprising the steps of: (a) performing impedance measurement between asensing electrode exposed to an atmosphere of a measurement gas and areference electrode exposed to a reference atmosphere, said sensingelectrode and said reference electrode being provided in said gassensor; (b) judging necessity of said recovering process based onelectrode reaction resistance in said gas sensor or a diagnosisparameter which is a parameter correlated with said electrode reactionresistance, said electrode reaction resistance and said diagnosisparameter being obtained based on a result of said impedancemeasurement; and (c), prior to said step (a), specifying a diagnosisfrequency which is a frequency of an alternating voltage used in saidimpedance measurement in said step (a), by performance of a preliminaryimpedance measurement in which an alternating voltage is applied betweensaid sensing electrode and said reference electrode with a frequencybeing varied within a predetermined frequency range in which a Bodediagram for a phase angle is allowed to be produced, wherein said step(a) and said step (b) are intermittently or periodically repeated duringuse of said gas sensor, and it is judged that said recovering process isnecessary when said diagnosis parameter satisfies a predeterminedthreshold condition in said step (b), in said step (a), said impedancemeasurement is performed by application of an alternating voltage atsaid diagnosis frequency, and in said step (b), it is judged that saidrecovering process is necessary when a value of a phase angle which isobtained by said impedance measurement is a value closer to 0° than apredetermined threshold.
 2. The gas-sensor diagnosing method accordingto claim 1, wherein in said step (c), a frequency contributing to anextremum in said Bode diagram is designated as said diagnosis frequency.3. A gas-sensor diagnosing method of judging necessity of a recoveringprocess carried out on a mixed-potential gas sensor for recovering anoutput of said gas sensor, said method comprising the steps of: (a)performing impedance measurement between a sensing electrode exposed toan atmosphere of a measurement gas and a reference electrode exposed toa reference atmosphere, said sensing electrode and said referenceelectrode being provided in said gas sensor; (b) judging necessity ofsaid recovering process based on electrode reaction resistance in saidgas sensor or a diagnosis parameter which is a parameter correlated withsaid electrode reaction resistance, said electrode reaction resistanceand said diagnosis parameter being obtained based on a result of saidimpedance measurement; and (c), prior to said step (a), specifying adiagnosis frequency which is a frequency of an alternating voltage usedin said impedance measurement in said step (a), by performance of apreliminary impedance measurement in which an alternating voltage isapplied between said sensing electrode and said reference electrode witha frequency being varied within a predetermined frequency range in whicha Bode diagram for an absolute value of impedance is allowed to beproduced, wherein said step (a) and said step (b) are intermittently orperiodically repeated during use of said gas sensor, and it is judgedthat said recovering process is necessary when said diagnosis parametersatisfies a predetermined threshold condition in said step (b), in saidstep (a), said impedance measurement is performed by application of analternating voltage at said diagnosis frequency, and in said step (b),it is judged that said recovering process is necessary when a value of acommon logarithm of an absolute value of impedance, which is obtained bysaid impedance measurement, is equal to, or smaller than, apredetermined threshold.
 4. The gas-sensor diagnosing method accordingto claim 3, wherein in said step (c), one of frequencies in the vicinityof a frequency contributing to a maximum of said value of a commonlogarithm of an absolute value of impedance in said Bode diagram isdesignated as said diagnosis frequency.
 5. The gas-sensor diagnosingmethod according to claim 1, wherein in said step (b), when saidrecovering process is carried out after it is judged that saidrecovering process is necessary, said threshold condition is re-setbased on a value of said diagnosis parameter which is providedimmediately after said recovering process.
 6. The gas-sensor diagnosingmethod according to claim 2, wherein in said step (b), when saidrecovering process is carried out after it is judged that saidrecovering process is necessary, said threshold condition is re-setbased on a value of said diagnosis parameter which is providedimmediately after said recovering process.
 7. The gas-sensor diagnosingmethod according to claim 3, wherein in said step (b), when saidrecovering process is carried out after it is judged that saidrecovering process is necessary, said threshold condition is re-setbased on a value of said diagnosis parameter which is providedimmediately after said recovering process.
 8. The gas-sensor diagnosingmethod according to claim 4, wherein in said step (b), when saidrecovering process is carried out after it is judged that saidrecovering process is necessary, said threshold condition is re-setbased on a value of said diagnosis parameter which is providedimmediately after said recovering process.